There is currently a large amount of attention being paid to the use of additive materials in cement in order to maintain or increase the strength of the cement while reducing the overall energy required to produce the material. In practice, a number of natural and manufactured materials are added to clinker in order to reduce the need for clinker minerals in the cement. These materials include limestone, waste slag from the manufacture of steel and iron, and naturally occurring pozzolan. Disadvantages exist to the use of these materials in practice. Quality concerns limit the introduction of limestone, as limestone naturally provides little to the strength of the finished product. Certain types of slag can be utilized positively for the introduction of strength to cement, but as a waste product of the manufacture of other compounds, the slag often does not have a consistent chemistry. Slags can also contain large amounts of free iron, which can cause premature wear of grinding elements used in the manufacture of cement. Pozzolan provides positive strength development in finished cement, but as a naturally occurring material, is not generally available in locations where the primary raw materials used in the manufacture of cement are mined.
In recent years, a number of processes have gained prominence in the production of artificial pozzolan from the calcining of clay. The manufacture of artificial pozzolan requires lower temperatures and less energy than the production of cement clinker, and is therefore gaining importance among cement manufacturers for its lower cost of production, as well as the positive effects of producing lower emissions (particularly CO2).
In practice, however, while the chemistry may be consistent with a positive effect on strength development, the production of these artificial pozzolans may create materials which are colored differently than the clinker used in the manufacture of cement. This is problematic where the color of the finished product is an important concern, such as when multiple sources of cement may be used for a single project. These issues with the coloration of the final product serve to limit the introduction of these synthetic pozzolans in the production of cement.
Therefore, it is an object of the present invention to provide a method for producing synthetic pozzolan having desired color characteristics, and in particular having a light grey color that many cement producers find desirable.